Constructors

The StaggeredGridView follow the same constructors convention than the GridView.
There are two more constructors: countBuilder and extentBuilder. These constructors allow you to define a builder for the layout and a builder for the children.

Tiles

A StaggeredGridView needs to know how to display each tile, and what widget is associated with a tile.

A tile needs to have a fixed number of cell to occupy in the cross axis.
For the extent in the main axis you have 3 options:

You want a fixed number of cells => use StaggeredTile.count.

You want a fixed extent => use StaggeredTile.extent.

You want a variable extent, defined by the content of the tile itself => use StaggeredTile.fit.